A. Field of the Invention
This invention relates to an apparatus and process for use in electrostatic copying machines, and more particularly, to an apparatus and process for depositing electrostatically and magnetically responsive particulate matter on a moving photoconductive surface bearing a latent electrostatic image.
B. Discription of the Prior Art
In typical electrostatic photocopying processes a surface of a photoconductive insulating material is charged with a uniform electrostatic charge. The charged surface of the photoconductive material is then exposed to a light image through a photographic transparency or some other suitable means. The portions of the charged surface that are irradiated by light are discharged while the remaining portions of the charged surface maintain their charge to form a latent electrostatic image that corresponds to the light image. The latent electrostatic image on the surface is developed by applying electrostatically responsive powder to the surface. The powder image thus formed is fixed directly to the photoconductive surface by fusing or the like.
More recently, magnetic forces and electrostatic forces have been used to develop latent electrostatic images in electrostatic photocopying machines. In some instances, the developer powder is applied to the conductive surface by using the well known "magnetic brush" technique. The developer powders that have been employed are well known in the art and generally comprise dyed or colored pigmented thermoplastic powders referred to as "toner" that are mixed with more course particles known as "carriers", such as, for example, iron filings. Developer powders can be formulated so that the toner carries a negative or positive charge. A typical positive developer powder is formulated from carbon black pigmented, polystyrene resin toner mixed with iron magnetites or ferrites. In any case, the toner and the carrier are selected so that the toner particle acquires the proper charge with respect to the latent electrostatic image. When the "developer brush" is brought into contact with the conductive surface greater attractive electrostatic forces of the charged image cause the toner particles to leave the carrier particles and adhere to the conductive surface.
Apparatus for using the magnetic brush technique are well known in the art. Typically a magnetic brush consists of a non-magnetic rotatably mounted cylinder having magnets fixed inside the cylinder. The cylinder is adapted to rotate with a portion of its surface immersed in a hopper having a supply of developer powder. The developer powder, being a mixture of iron carrier particles and electrostatic toner particles, is magnetically attracted to the surface of the cylinder to form a brush-type arrangement thereon as a result of the magnetic flux developed by the magnets. The bristles of the brush conform to the lines of magnetic flux. The conductive surface such as, for example, a sheet of paper bearing a latent electrostatic image is brought into physical contact with the brush and toner is thereupon deposited on the sheet of paper.
The rotating cylinder continues to attract developer powder and returns part or all of this material to the hopper within one revolution. Accordingly, a fresh mix is always available to the copy sheet surface at its point of contact with the brush.
In every instance the systems and apparatuses of the prior art require a delicate balance between the ratio of iron carrier particles and the electrostatic toner particles as well as an intimate admixture of uniform quality. Quite often variations in the ratio of the inro carrier particles to the electrostatic toner particles is experienced resulting in poor coverage of the image to be developed. Furthermore, the iron carrier particles gradually deteriorate and frequently the entire system must be cleaned and replaced with a fresh admixture of carrier and toner.
It is well recognized that the step of developing the latent electrostatic image is perhaps the most critical step in all of the process steps of electrostatic copying. The final print quality can be no better than the quality of development step. Recently significant improvements have developed in the method of image developing, and particularly, a new developer powder has been developed as shown for example in U.S. Pat. No. 3,639,245 involving a composite developer powder comprising magnetizable particles embedded in the toner particles. The composite developer powder of the above mentioned patent is used with the prior art apparatuses such as, for example, the brush-type applicators that use the rotating cylinder to carry the developer powder from its supply to the conductive surface.
Despite this improvement in developer powders there is still a need for improvements in the devices employing such powders. For example, the cylinder being journalled to a shaft for rotation developes considerable misalignment between the cylinder and the conductive surface resulting in a poor nonuniform deposition of developer powders to the surface. Such failures result in customer complaints and considerable expense in replacing the cylinders and the like. Furthermore, the prior art devices are complex; a need for simplifying the structures utilizing the brush-type technique and to obtain savings in the cost of manufacture of such devices without sacrificing performance dependability are needed.
Accordingly, I have developed an apparatus and process that substantially eliminates the number of moving parts and, particularly, the necessity for the rotating cylinder of the prior art devices. My device and process being of a less complicated structure is considerably less expensive to manufacture than the prior art devices. My apparatus and process using the improved magnetically and electrostatically responsive developer powders is capable of developing latent electrostatic images of greater clarity and resolution than the prior art devices.